1. Field of the Invention
The present invention relates to an endoscope system and method of operating the endoscope system. More particularly, the present invention relates to an endoscope system and method of operating the endoscope system, in which light sources of plural colors are combined for illuminating an object in a body cavity, and an endoscopic image of the object can be formed with acceptable image hue even with light of the light sources without use of continuous spectrum light from a broadband light source.
2. Description Related to the Prior Art
An endoscope system is well-known in the field of medical diagnosis, and includes a light source apparatus, an endoscope and a processing apparatus. The light source apparatus generates light for illuminating an object of interest in a body cavity, for example, mucosa or tissue. A xenon lamp for generating continuous spectrum light or broadband light has been widely used in the light source apparatus as a light source. However, use of semiconductor light sources, such as LEDs (light emitting diodes), has been developed instead of the xenon lamp as a broadband light source. For example, blue, green and red LEDs and the like are combined for emitting light of different colors in the light source, so that polychromatic light or multi color spectrum light is generated for illumination with an emission spectrum of the combination of light components of those colors.
U.S. Pat. Pub. No. 2015/092,035 (corresponding to JP-A 2013-255655) discloses the endoscope system in which the light source apparatus includes four semiconductor light sources controllable discretely. The emission spectrum (intensity distribution for respective wavelengths) of light is adjusted by controlling intensities of the semiconductor light sources, so that an object of interest is illuminated with light optimized in consideration of a characteristic of an image to be formed. Examples of the characteristics of the image are an image with a high dynamic range in relation to brightness, an image having a low color temperature, an image having a high color temperature, an image of an object with a small area illuminated with special narrow band light of a narrow wavelength range for special use, and the like. The emission spectrum is adjusted according to the respective characteristics.
Also, JP-A 2013-202166 discloses the semiconductor light sources controllable discretely from one another, in combination with a detector for detecting an endoscope type of the endoscope. A driving condition for the semiconductor light sources is determined by considering the endoscope type. A light guide device for transmitting light for illumination has a transmission characteristic with a difference between a plurality of endoscope types. Thus, an intensity ratio between intensities of the semiconductor light sources is determined according to the transmission characteristic of the light guide device by recognizing the endoscope type.
For illumination in the endoscope system, the use of the polychromatic light from a plurality of semiconductor light sources has been spread recently in the field of endoscopy rather than the use of the continuous spectrum light from the xenon lamp utilized conventionally. However, there is a difference in the emission spectrum between the continuous spectrum light and the polychromatic light. Even after imaging of the same object of interest, there occurs a difference in image hue (visual form) of the object of interest between an image after lighting with the continuous spectrum light and an image after lighting with the polychromatic light. Both of the continuous spectrum light and the polychromatic light are available for use in lighting with respective advantages in spite of the difference in the image hue of the object of interest. However, the use of the polychromatic light for imaging is particularly advantageous, because the semiconductor light sources are controllable discretely to adjust the emission spectrum appropriately for a type, condition or the like of the object of interest.
In the field of the endoscope system, a history of using the continuous spectrum light the xenon lamp and the like for illumination is long considerably. Most of doctors or operators of endoscopy are accustomed to the image hue of the object of interest illuminated with the continuous spectrum light of the xenon lamp. It is preferable to image the object of interest in the same manner as with illumination of the continuous spectrum light of the xenon lamp widely used even in the use of the polychromatic light of a plurality of the semiconductor light sources for illumination. Furthermore, numerous archived endoscopic images for medical database have been originally recorded by use of the continuous spectrum light of the xenon lamp. It is preferable newly to image the object of interest in the same manner as with illumination of the continuous spectrum light of the xenon lamp widely used even in the use of the polychromatic light of a plurality of the semiconductor light sources, because of facilitating comparison between cases and making decision of diagnosis.
In view of this situation, color rendering of the emission spectrum of the continuous spectrum light by use of the semiconductor light sources has been attempted technically in the field of endoscopy. However, it is extremely difficult to output light of the emission spectrum of the continuous spectrum light by use of the semiconductor light sources reliably in a complete manner. For example, a blue LED (light emitting diode) and a green LED are used as light sources. The blue and green LEDs have such a characteristic that an intensity decreases from a center wavelength of the wavelength range toward wavelengths different from the center wavelength. The intensity of an intermediate color with a wavelength between blue and green is difficult to adjust only by adjusting the intensity of the blue and green LEDs. Assuming that the intensity of the center wavelength of each of the blue and green LEDs is set near to the intensity of the continuous spectrum light, the intensity of the intermediate color between blue and green becomes considerably smaller than the intensity of the continuous spectrum light. Assuming that the intensity of each of the blue and green LEDs is increased to set the intensity of the intermediate color between blue and green near to the intensity of the continuous spectrum light, the intensity of the center wavelength of each of the blue and green LEDs may become extremely larger than the intensity of the continuous spectrum light.